1. Field of the Invention
The present invention relates to an apparatus and a method utilized in optical communications using an optical transmission medium.
2. Description of the Related Art
Optical communications utilizing optical fibers have been carried out for increasing data transmission speed between computers, between a computer and its peripherals or between other digital devices. For such optical communications utilizing optical fibers, the two-fiber bidirectional communications scheme is generally used wherein two optical fibers are used. However, an apparatus for implementing the two-fiber bidirectional communications scheme is required to have a large configuration and the cost of the apparatus is high since two optical fibers are required and a device size of means for emitting signal light and so on is large.
Therefore, the single-fiber bidirectional communications scheme has been developed wherein a single optical fiber is used. An optical transmitter-receiver will now be described as an example of an optical communications apparatus used for the single-fiber bidirectional communications. The optical transmitter-receiver is coupled to a tip of an optical fiber and transmits and receives signal light.
FIG. 1 shows an example of the optical transmitter-receiver used for the single-fiber bidirectional communications. FIG. 1 is a side view of the main part of the optical transmitter-receiver. The transmitter-receiver comprises: a semiconductor substrate 101 formed of silicon semiconductor or gallium arsenide (GaAs) semiconductor and in an upper surface of which a photodiode 102 as a light receiving means is formed; a prism 103 joined to the surface of the substrate 101; a semiconductor device 104 in the shape of rectangular solid joined to the surface of the substrate 101; a laser diode 105 as a light emitting means joined to the top of the semiconductor device 104; and a lens 107 for allowing first signal light L.sub.1 emitted from the laser diode 105 to be sent to another optical transmitter-receiver to enter an end face of an optical fiber 106 as a communications line and for condensing second signal light L.sub.2 sent from the other transmitter-receiver through the optical fiber 106 and emitted from the end face of the optical fiber 106 and introducing second signal light L.sub.2 to the photodiode 102.
On the substrate 101 the prism 103 is placed on the photodiode 102. The semiconductor device 104 is placed on a side of the prism 103. The laser diode 105 is arranged such that first signal light L.sub.1 is emitted towards the prism 103. The prism 103 has a slope forming an angle of 45 degrees with the upper surface of substrate 101, for example, on a side thereof facing the laser diode 105. A half mirror 103a is formed on the slope. For the optical fiber 106, a large-diameter plastic fiber may be used.
In the optical transmitter-receiver with such a configuration, the laser diode 105 is driven by a drive circuit not shown and first signal light L.sub.1 is emitted from the laser diode 105. First signal light L.sub.1 enter the half mirror 103a of the prism 103 with a numerical aperture (NA) of 0.1, for example, where nearly 50 percent of quantity of light, for example, is reflected to enter the lens 107. First signal light L.sub.1 is condensed by the lens 107 and enters the optical fiber 106 with a numerical aperture of 0.1, for example. An numerical aperture of first signal light L.sub.1 emitted from the laser diode 105 depends on the laser diode 105.
On the other hand, second signal light L.sub.2 sent from the other optical transmitter-receiver through the optical fiber 106 is emitted from the optical fiber 106 with a numerical aperture of 0.3, for example. Second signal light L.sub.2 is condensed by the lens 107 so that the numerical aperture is 0.3 and enters the half mirror 103a of the prism 103. Nearly 50 percent of quantity of light passes through to enter the photodiode 102. The light is then transformed to an electric signal. An numerical aperture of second signal light L.sub.2 emitted from the optical fiber 106 depends on the optical fiber 106.
In the optical transmitter-receiver shown in FIG. 1 as an example of optical communications apparatus, however, nearly 50 percent of quantity of light is lost in each of first signal light L.sub.1 and second signal light L.sub.2. Therefore an optical coupling efficiency and a light receiving efficiency are reduced. In the related art techniques as described so far, it is difficult to improve both light coupling efficiency of a light emitting means such as a laser diode to an optical fiber and light receiving efficiency of a light receiving means such as a photodiode for receiving signal light.
Another problem in the related art techniques is difficulty in removing stray light components, that is, signal light generated by a light emitting means reflecting off an end face of an optical fiber and entering a light receiving means. Such stray light components affect optical communications.
Furthermore, an optical system including an image-formation lens is placed between light emitting and receiving means and an optical fiber in related art techniques. Consequently the number of parts included in the apparatus increases and the configuration of the apparatus is complicated and large in size.